Isocyanates have been industrially widely used as raw materials for, for example, polyurethane or polyurea. Conventionally, isocyanates have been industrially produced by a reaction of an amine with phosgene (phosgene method). However, phosgene has various problems such as high toxicity, complicated handling, and further, the necessity of special consideration for corrosion of apparatuses caused by a large amount of hydrochloric acid generated as a by-product from phosgene. Accordingly, it has been desired to develop a method for industrially producing an isocyanate, which will replace the aforementioned phosgene method.
As a method for producing an isocyanate without using phosgene, a method for producing an isocyanate by performing pyrolysis on a carbamate obtained by reacting an amine, urea and/or an N-unsubstituted carbamic acid ester, and alcohol (carbamation reaction) has been known (urea method), for example. With regard to the production of an isocyanate according to this method, it has been known that gaseous ammonia is generated as a by-product, and that high-boiling components are generated after the recovery of an isocyanate. Thus, a method for recovering and treating such components has been studied.
In the case of the carbamation reaction performed in the urea method, ammonia is generated as a by-product. Since ammonia is used as a raw material for basic chemicals such as nitric acid and nitrogen fertilizers such as ammonia sulfate, it is an industrially extremely important substance. Ammonia is used in the form of liquid ammonia or ammonia water.
As a method for recovering ammonia generated as a by-product in the production of a carbamate, there has been proposed, for example, a method which comprises washing exhaust gas containing an organic matter, carbon dioxide and ammonia with an alkaline solution (e.g. caustic soda solution) to remove the organic matter and the like, and at the same time, distilling the ammonia as a top product (see, for example, Patent Literature 1 below). In addition, Patent Literature 2 discloses a method which comprises washing alcohol contained in ammonia generated as a by-product with a small amount of water and recycling it.
On the other hand, with regard to high-boiling components obtained after the recovery of the isocyanate, for example, Patent Literature 3 discloses a method which comprises separating isocyanates from an organic residual matter containing diisocyanates at a specific temperature and under a specific pressure, and carrying the residual matter out of the reaction system according to forced transfer.